Update gz-sensors to use new message generation pipeline

[mjcarroll]

Allows gz-sim to take advantage of the new message generation pipeline in gazebosim/gz-msgs#339

[iche033]

Got a couple of compile warnings from the generated header file which look like false positives:

In file included from /home/iche/code/gz_h_ws/build/gz-sensors8/gz_msgs_gen/gz/msgs/performance_sensor_metrics.pb.h:10,
                 from /home/iche/code/gz_h_ws/src/gz-sensors/src/Sensor.cc:24:
/home/iche/code/gz_h_ws/build/gz-sensors8/gz_msgs_gen/gz/msgs/details/performance_sensor_metrics.pb.h:402:20: warning: ‘simUpdateRate’ may be used uninitialized in this function [-Wmaybe-uninitialized]
  402 |   sim_update_rate_ = value;
      |   ~~~~~~~~~~~~~~~~~^~~~~~~
/home/iche/code/gz_h_ws/src/gz-sensors/src/Sensor.cc:348:10: note: ‘simUpdateRate’ was declared here
  348 |   double simUpdateRate;

This fixes the warnings for me:

diff --git a/src/Sensor.cc b/src/Sensor.cc
index 242b02d..0605de1 100644
--- a/src/Sensor.cc
+++ b/src/Sensor.cc
@@ -345,8 +345,8 @@ void SensorPrivate::PublishMetrics(const std::chrono::duration<double> &_now)
   }
 
   // Computes simulation update rate and real update rate.
-  double simUpdateRate;
-  double realUpdateRate;
+  double simUpdateRate = 0.0;
+  double realUpdateRate = 0.0;
   const auto clockNow = std::chrono::steady_clock::now();
   // If lastUpdateTime == 0 means it wasn't initialized yet.
   if (this->lastUpdateTime.count() > 0)